Inductive power coupling allows energy to be transferred from a power supply to an electric load without a wired connection therebetween. An oscillating electric potential is applied across a primary inductor. This sets up an oscillating magnetic field in the vicinity of the primary inductor. The oscillating magnetic field may induce a secondary oscillating electrical potential in a secondary inductor placed close to the primary inductor. In this way, electrical energy may be transmitted from the primary inductor to the secondary inductor by electromagnetic induction without a conductive connection between the inductors.
When electrical energy is transferred from a primary inductor to a secondary inductor, the inductors are said to be inductively coupled. An electric load wired in series with such a secondary inductor may draw energy from the power source wired to the primary inductor when the secondary inductor is inductively coupled thereto.
The strength of the induced voltage in the secondary inductor varies according to the oscillating frequency of the electrical potential provided to the primary inductor. The induced voltage is strongest when the oscillating frequency equals the resonant frequency of the system. The resonant frequency fR depends upon the inductance L and the capacitance C of the system according to the equation
      f    R    =            1              2        ⁢        π        ⁢                  LC                      .  
Known inductive power transfer systems typically transmit power at the resonant frequency of the inductive couple. This can be difficult to maintain as the resonant frequency of the system may fluctuate during power transmission, for example in response to changing environmental conditions or variations in alignment between primary and secondary coils.
Inductive transfer systems designed to transmit at resonance therefore require tuning mechanisms for maintaining transmission at the resonant frequency of the system. Tuning may be achieved by adjusting the driving frequency to seek resonance. For example, U.S. Pat. No. 6,825,620, titled “Inductively coupled ballast circuit” to Kuennen et al. describes a resonance seeking ballast circuit for inductively providing power to a load. The ballast circuit includes an oscillator, a driver, a switching circuit, a resonant tank circuit and a current sensing circuit. The current sensing circuit provides a current feedback signal to the oscillator that is representative of the current in the resonant tank circuit. The current feedback signal drives the frequency of the ballast circuit causing the ballast circuit to seek resonance. The ballast circuit includes a current limit circuit that is inductively coupled to the resonant tank circuit. The current limit circuit disables the ballast circuit when the current in the ballast circuit exceeds a predetermined threshold or falls outside a predetermined range.
Alternatively, tuning may be achieved by adjusting the characteristics of the inductive system. For example, U.S. Pat. No. 2,212,414, titled “Adaptive inductive power supply” to Baarman describes a contactless power supply which has a dynamically configurable tank circuit powered by an inverter. The contactless power supply is inductively coupled to one or more loads. The inverter is connected to a DC power source. When loads are added or removed from the system, the contactless power supply is capable of modifying the resonant frequency of the tank circuit, the inverter frequency, the inverter duty cycle or the rail voltage of the DC power source.
Tuning mechanisms such as those described above are necessary in order to maintain transmission at resonance because resonant transmission is highly sensitive. At resonance small variations to the system result in large changes to the power transferred. A further problem associated with resonant transmission is the high transmission voltages involved. At high operating voltages, the capacitors and transistors in the circuit need to be relatively large.
There is a need for an inductive transfer system with a higher tolerance to environmental fluctuations and variations in inductive coil alignment and which transmits at low voltages. The present invention addresses this need.